Detection apparatus and techniques for monitoring the rate of change of thickness of a transparent layer using light interference are well known. Such techniques are utilized to measure the rate of etching of a layer of silicon dioxide upon a silicon substrate, for example. By these techniques a light beam is directed upon the transparent layer and reflected from both the surface of the transparent layer and from the surface of its underlying opaque silicon layer. The reflections interfere with one another causing a beam of varying light intensity that can be detected as the thickness of the oxide layer changes. When the trace goes through one period of oscillation the thickness of the transparent layer is changed by .lambda./2n where .lambda. is the wavelength of the light and n is the refractive index of the transparent layer.
Other systems have been developed for determining the rate of change of thickness of opaque materials such as silicon or metal. For example, copending application Ser. No. 811,741 to Habegger for "Interferometric Process and Apparatus for the Measurement of the Etch Rate of Opaque Surfaces" describes such a system. The Habegger invention is satisfactory for measuring the etch rate of opaque materials but has certain limitations. For example, the Habegger arrangement requires the use of a beam path compensator to equalize the paths of the two high intensity beams returning from the uncovered opaque surface and the opaque surface underlying the transparent layer. The present invention is an improvement thereon since it provides a different arrangement of beam splitters which provide equal beam path lengths that do not require a path length compensator. Moreover, the present invention provides a capability for visual observation of the area being monitored.